What is the difference between a homeless man who claims to speak to God and a saint who says the same? When I posed this question to Andrew Scull, the author of the recent book Madness in Civilization: A Cultural History of Insanity, he chuckled and cited a quip by the philosopher Bertrand Russell: “From a scientific point of view, we can make no distinction between the man who eats little and sees heaven and the man who drinks much and sees snakes.”

When Russell said it, it was an atheist’s diss against mystical visions, and against religion generally. Coming from Scull, it’s more of an invitation to explore the complicated relationship between religion and madness over thousands of years of cultural history. Scull argues that stories of the supernatural—often paired with stories of madness—have long been a source of power for religious organizations, proof of their authority to interpret the presence of the divine—and evil—on earth.

Over time, though, the relationship between religion and madness has become more ambiguous. Science has transformed the way many modern believers and religious institutions approach faith. For a long time, the influence of God or Satan was a sufficient explanation for all sorts of phenomena, from so-called possessions to the kinds of visions supposedly experienced by Catherine of Siena or Teresa of Avila. Now, those who decode visions and possessions are psychiatrists, not priests, and explanations are rooted in the individual mind, not the interference of God or the devil.

In the deserts of ancient Israel, a homeless man who was said to have visions and perform miracles was revered by some as the son of God. Which leaves two questions: How should claims of divine encounters be interpreted in the modern world? And if a homeless man on the street were actually the messiah, would he be recognized?

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As it turns out, being a biblical prophet isn’t a foolproof shield against accusations of insanity. “If you look at the history of the Old Testament prophets, the answer is hard to disentangle,” Scull explained. “Many parts of their lives are seen as mad or possibly possessed, and only over time do some of them get reinterpreted as inspired.” Throughout biblical times, apparent mental disturbances were often seen as divine punishments or demonic possessions. Rarely, they were understood as heavenly visions. But even this boundary was fluid. “Saul is seen at one point as behaving like a prophet, then later on people see him as not entirely right in the head, attributing that to God punishing him for not slaughtering everybody when he was supposed to,” Scull said.

One of the main points of Scull’s book is that interpretations of madness have changed dramatically over centuries and across cultures. It’s unclear, for example, whether manifestations of madness in ancient times would be recognizable as mental illness today. But Scull does offer a sort of common-sense definition of how instances of madness have been identified over time: disruptive behavior, disturbed perception, loss of speech, diminished emotional or rational control. Especially in the early centuries A.D., he writes, identifying and curing this kind of behavior helped Christianity spread across the eastern Mediterranean and, eventually, the Roman Empire. During this time, healings and exorcisms were a common preliminary rite before baptisms. Just as Jesus had demonstrated a power to cast out demons, so early Christian leaders claimed to rid people of demonic possession—or lead them away from sin, if their madness was seen as punishment from an angry God.

Divine visions were also an important part of how Christians proved their spiritual authority and claimed to distinguish their religion from that of pagans (although the legitimacy of the visions depended heavily on who was having them, and who was interpreting them). “The presence of miracles and saints was an important calling card of Christians—that’s the way they converted pagan Europe,” Scull said. Christian leaders collected and distributed the body parts of saints and martyrs as relics and urged laymen to pray to them for divine intercession. They believed certain people who had divine visions could produce demonstrable miracles, including ridding people of supposed possessions, and those powers were thought to linger after their death. “Typically, saints whose martyrdom had involved beheading or some damage to the head—those were the saints who seemed to have particular power over mental disturbance,” Scull said.

In the Middle Ages, as medical practice began to develop, cases of madness were often treated as both spiritual and physical. “Physicians conceded that some cases of madness were really possessions, or things that belonged to divine,” Scull said. Those cases might be referred to priests. But others, the doctors argued, “were theirs, because they were rooted in the body, in the humors.”

In Europe, things started to change in the 16th century. These were the first days of the Scientific Revolution and the so-called Age of Reason, marked by the rise of rational philosophy and scientific invention. The Catholic Church was still an incredibly powerful force in European culture, but it also wasn’t the same institution as 10 centuries prior. Powerful challenges to the Church’s authority had spread through Western Europe. During and following the Protestant Reformation, accusations of madness were often used as a form of political power, used to argue for the legitimacy of one Christian denomination over another.

In the 1760s and 70s, for example, an Austrian priest named Johann Joseph Gassner started performing exorcisms across southwestern Germany, treating everything from “blindness, manic propensities to dance … the epileptic, the lame … the hysteric, and the crazed.” Protestants ridiculed these alleged healings as vestiges of Catholic superstition, and some Church leaders felt sensitive to this criticism. Others cautioned against the political instability that might accompany rising fears of demons and possessions. Eventually, Pope Pius VI ordered the priest to stop performing the exorcisms. “The Pope himself surely did not kill off popular beliefs in the Devil and possession,” Scull writes, “but [this incident] did indicate the degree to which polite society was distancing itself from older religious accounts of illness and suffering, and of madness in particular.”

Yet accusations of madness helped other kinds of Protestantism spread. Throughout the 16th and 17th centuries, accusations of witchcraft and possession were powerful tools used by rival Christian groups, Scull said. For example, the preacher John Wesley, who led a Methodist revival in England in the late 18th century, was a “forceful advocate of spiritual health of the mentally disturbed through communal rituals of fasting and prayer.”

By the 19th century, particularly in Western Europe, many doctors who studied mental illness began to push back against divine interpretations of visions. A famous French neurologist, Jean-Martin Charcot, claimed that all Christian saints were hysterics, experiencing mystical visions that were really just signs of underlying pathology. This, Scull said, was part of a deeper polemical argument against religion’s power in society. By that time, “most hospitals in France, and most of those ministering to the mentally ill, were the religious orders,” he said. “There’s a big fight under the surface through the 19th century—an attempt to laicize those institutions, and make medicine free of the competing interests of clerics. And that’s felt nowhere more profoundly than in the question of madness.”

Asylums began acquiring medical staff. “Madness,” as a term, fell out of favor, and was seen as a slur on the sick. This was a clear turning point in the way insanity was understood in the West: Madness started to be understood as an affliction of body, not the spirit.

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The transition over time from “madness” to “mental illness” is a linguistic tweak that accounts for a larger metaphysical reshuffling. This is a recurring theme in Scull’s work, which largely focuses on the West: Insanity, he argues, is culturally dependent, as is the language used to describe it. He does make some references to how madness was treated in non-Western cultures, and these reinforce his broader point: The words people use to describe mental illness say a lot about how they understand the nature of existence.

In early Islam, for example, prophetic healings for insanity included prayers, bloodletting, and “cauterization of the head with hot irons.” This last technique was used, Scull writes, because demons and spirits were believed to be afraid of iron. Not coincidentally, the Arabic word for “genie” or “spirit” is jinn—the word from which majnoon, or “crazy,” is derived.

Until very recently in China, “madness was never interpreted as a distinct illness, but was instead, like other forms of ill-health, seen as deriving from a more comprehensive corporeal and cosmological imbalance,” Scull writes. The words mostly commonly used to describe possession and mental confusion, kuang, feng, and dian, were common-sense descriptions of symptoms (such as loss of verbal or emotional control), rather than an ongoing state of being mad or insane.

In the decades following World War II, the focus of psychiatry shifted away from the psychoanalytic techniques advocated by Freud and his followers and toward biological factors and causes. As Steven Sharfstein, the former president of the American Psychiatric Association, put it, public discourse around mental illness transformed from “the biopsychosocial model” to “the bio-bio-bio model.” This shift matters for the relationship between religion and madness. Even those who remain deeply religious in the U.S. or other developed countries likely accept some version of the biological view of mental illness. Some religious leaders have been vocal in their acceptance of this; in a 1996 speech, for example, Pope John Paul II called mental illness “the most absurd and incomprehensible [affliction].”

Yet for some, religion still plays an important role in treatment: A 2003 study published by Health Services Research found that more people reach out to members of the clergy about mental-health issues than to psychiatrists or doctors. And some traditions also remain actively open to evidence of divine influence in everyday life. In 2012, for example, roughly a quarter of American religious congregations reported instances of members speaking in tongues, a five-percentage-point bump from what was reported in 1998. But even prominent religious institutions have faced awkwardness in their teachings about possessions and visions. Pope Francis often speaks about the influence of Satan, and last summer, he officially recognized the International Association of Exorcists, a small group of roughly 250 priests who still follow the practice of casting out demons. On the other hand, the Vatican’s process for legitimizing miracles and canonizing saints is as close to technocratic and rational as it can get while still involving supernatural encounters: Medical miracles, for example, often go through years-long investigations by multiple panels of local clergymen, doctors, theologians, and Vatican advisors.

As “madness” became “mental illness,” the role of religion in explaining out-of-the-ordinary behavior has faded significantly, and medicine has taken its place. It’s not that strange happenings have faded from importance in religious life; it’s that in the shadow of modern medicine, it’s more difficult to discern between the strange phenomena of the brain and the potentially stranger phenomena of the supernatural.

Which brings back the initial question: In a time of prescriptions and psychiatry and an ever-increasing focus on the brain, what’s the difference between a homeless man who talks to God and a saint who does the same? To a skeptic, the question is irrelevant; both are equally insane. Scull himself is not necessarily interested in this question; he hinted that he’s not personally inclined toward religious belief. But for the faithful, the difference between the homeless man and the saint may be one of history, more than anything; the homeless man suffers the distinct disadvantage of talking to God today, rather than 10 centuries ago. Stories of visions and possessions seem to be of an earlier time, when madness was seen as just another manifestation of the power of God, and the devil. As such, the consequences of modern psychiatry are intellectually different for those who believe and those who don’t. Scientifically minded believers may accept that mental illness is a medical issue, but that may also make it more difficult to accept the possibility of a divine vision. For non-believers, seeing God is the disease. But the faithful risk something greater: mistaking Him for disease.

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Emma Green is a staff writer at ​The Atlantic, where she covers politics, policy, and religion.